Re: [PATCH V1 0/6] fast qom tree get

Using qom-list and qom-get to get all the nodes and property values in a QOM tree can take multiple seconds because it requires 1000's of individual QOM requests. Some managers fetch the entire tree or a large subset of it when starting a new VM, and this cost is a substantial fraction of start up time.

To reduce this cost, consider QAPI calls that fetch more information in each call: * qom-list-get: given a path, return a list of properties and values. * qom-list-getv: given a list of paths, return a list of properties and values for each path. * qom-tree-get: given a path, return all descendant nodes rooted at that path, with properties and values for each.

In all cases, a returned property is represented by ObjectPropertyValue, with fields name, type, value, and error. If an error occurs when reading a value, the value field is omitted, and the error message is returned in the the error field. Thus an error for one property will not cause a bulk fetch operation to fail.

To evaluate each method, I modified scripts/qmp/qom-tree to use the method, verified all methods produce the same output, and timed each using: qemu-system-x86_64 -display none \ -chardev socket,id=monitor0,path=/tmp/vm1.sock,server=on,wait=off \ -mon monitor0,mode=control & time qom-tree -s /tmp/vm1.sock > /dev/null

I only measured once per method, but the variation is low after a warm up run. The 'real - user - sys' column is a proxy for QEMU CPU time. method real(s) user(s) sys(s) (real - user - sys)(s) qom-list / qom-get 2.048 0.932 0.057 1.059 qom-list-get 0.402 0.230 0.029 0.143 qom-list-getv 0.200 0.132 0.015 0.053 qom-tree-get 0.143 0.123 0.012 0.008 qom-tree-get is the clear winner, reducing elapsed time by a factor of 14X, and reducing QEMU CPU time by 132X. qom-list-getv is slower when fetching the entire tree, but can beat qom-tree-get when only a subset of the tree needs to be fetched (not shown). qom-list-get is shown for comparison only, and is not included in this series.